Getting Started

Understanding the Simulator Design

A website called Trailing Edge dot com has created a generic simulator that can be configured to mimic almost any computer. They have a couple dozen old DEC and IBM computers running, as well as the DG Nova.

No Windows Install process is required. The simulator consists of just a single .exe file that opens a command window similar to DOS. The exe file creates the proper environment to interpret the machine code for a given computer, but does not contain any actual machine code. In the case of the nova.exe it can be thought of as a Nova CPU with a console for a front panel. The panel understands a set of commands that allow you to define the hardware devices available to the CPU, which type of Nova computer you want it to emulate, and so on.

Most of the hardware “devices” you can connect are represented by files on the Windows system. So if you have a file that contains an image of an IRIS Logical Unit zero, you can ATTACH it to the simulator as its Disk Pack Drive zero (with the proper syntax) and then issue a BOOT command to run IRIS. Or you could instead attach an image of a Data General RDOS drive and boot that ( Trailing Edge has a working RDOS drive ).

How to run IRIS on Windows

Here’s the Shortest Path

  1. Register on this site in order to access the IRIS drive file (step 6).
  2. Create a working directory /iris/ on your computer.
  3. Navigate to this SimH page and click on the last link for Windows executables. There is a separate exe file for every old computer that simH can simulate. They are all packaged in a single zip file.
  4. Click the Download button and then unzip the file.
  5. Move nova.exe to /iris/  (you can delete everything else).
  6. Navigate to this download page and download the Diablo 31 Logical Unit zero (or alternately, the Century C114 unit zero).
  7. Unzip the file and move all contents to your /iris/ directory.
  8. Navigate to your /iris/ directory.
  9. Double-click nova.exe to launch the simulator.
  10. At the sim> prompt, type DO IRIS-D31.SIM (or IRIS-C114.SIM).
  11. IRIS should boot.
  12. At BAD BLOCK prompt, press <return>.
  13. At Date and Time prompt, enter a backslash.
  14. Press <esc> to get a login prompt.
  15. Type <Ctl-P> to disable parity (remember that?).
  16. Type uppercase MANAGER to log in as system manager.
  17. You should have an IRIS system prompt ‘#’.
  18. Have fun! Browse our Downloads >> Documentation to recover old IRIS manuals to refresh your memory on what you can do.

Comments on commands inside IRIS-D31.SIM / IRIS-C114.SIM files.

  • The DO command executes a batch file of SimH commands.
  • Disk packs are predefined devices named DKP0, DKP1, DKP2, DKP3.
  • SET DKP0 command tells SimH that drive DKP0 will be a Diablo 31 (or C114).
  • ATTACH command connects a SimH device to a Windows file which is a binary image of an IRIS LU.
  • SET CONSOLE WRU=6 command alters the SimH escape command to <CTL-F> (see below).

Switching between IRIS and the Simulator

The main console can switch between two modes:
1. As PORT 0 on the IRIS system
2. As command console for the Simulator (simH).

When running as PORT 0, you can escape to the SimH console by typing <Ctl-F>. The Nova execution will pause indefinitely and you will get a ‘sim>’ prompt. (note that the default is <CTL-E> but we need that in IRIS).

When running as the SimH console, you can examine memory, detach or attach devices, and run any other SimH commands. To resume the Nova and IRIS, use the command ‘C’ (for Continue).

Alternate C114 System

As of last 2016, we also have IRIS running on a Century 114 system. This provides a lot more usable disk space. The above instructions could be amended to reference the C114 download instead of the D31 download.


There is a lot of info on how this simulator works. It is located on the simH website. Navigate to this SimH page and download the following docs for future reference.

  • nova_doc.doc  ( info specific to the nova implementation of simH )
  • simh_doc.doc  ( general info about simH )
  • simh_faq.doc
  • simh_breakpoints.doc  ( using simH breakpoints for debugging ASM programs ).

Known Issues:

  • The main console only runs in dumb terminal mode. If you want VT100 capability (or other) you will need a second connection to SimH via a Telnet connection. (see the Main Menu >>> Simulator >>> Terminal Emulator).
  • Managing a workable speed can be tricky. Most things work no matter what the speed settings are. But some issues can arise. See: Post on System Speed.

3 thoughts on “Getting Started

  1. David TakleDavid Takle Post author

    If IRIS executes a HALT instruction (which may happen after a TRAPFAULT), the simulator will drop out to the ‘sim>’ prompt. Most likely a Continue command ‘C’ will not help.
    You can reboot IRIS by typing ‘BOOT DKP0’.
    Rerunning the entire SimH startup script (rather than a simply BOOT) will produce errors when it tries to ATTACH devices that are already attached.

  2. Paul SchwinbergPaul Schwinberg

    I followed the instructions under getting started. After typing the DO IRIS-D31.SIM
    the simulator gives File open error.
    The D31 file is 2,257KB in size. I looked at the memory content of the simulated NOVA and it was all zero.
    Any help as to what I am doing wrong would be good, Thanks.


    1. David TakleDavid Takle Post author

      An empty drive usually indicates that the simulator created the drive, given the name that was in the simH command file.
      Probably the name in the command did not match the name of the DKP that was downloaded, so it did not ATTACH.

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